Integrated circuit chips are used in a multitude of different applications, many requiring some form of security for data stored on the chip. Typically, a means is provided to prevent data stored on the chip from being read by an external agent. Under certain circumstances it may also be desirable to prevent data from being written onto the chip. For example, data stored on the chip may be used as a key to decipher or decode a public signal such a scrambled satellite video signal. Even if the stored data can not be read, if the stored data can be overwritten by an external agent, the code may be broken by a systematic overwriting of the data and iterative comparisons to the scrambled signal.
A read only memory (ROM) is a typical memory device which can not be overwritten. However, ROMs are programmed during manufacture which can create security problems for the end user who may desire to keep its code confidential. A programmable read only memory (PROM) may be programmed by the end user, thereby avoiding confidentiality problems. However, a PROM is less secure since it maintains its contents when voltage is removed, allowing the contents to be microprobed. The code in a PROM can also be read with an electron beam. Furthermore, a PROM is more expensive to produce than a RAM.
To the extent that previous security measures include the use of I/O pads on the chip, such pads are most likely dedicated exclusively to the security function. As the need to provide additional signal lines to a chip increases, the use of I/O pads for limited or one-time security functions is undesirable. This is particularly true if the pads are needed only to initially secure the data on the chip and are not required for normal operations.